1. Technical Field
This invention relates to the manufacture of conductive pastes and more particularly to a method of predicting and controlling the viscosity of conductive pastes in order to provide manufacturing uniformity.
Conductive pastes comprising a conductive (typically metal) powder suspended in a carrier (typically an organic vehicle), are widely used in the fabrication of electronic components. For example, conductive molybdenum paste is used to fabricate multilayer ceramic integrated circuit substrates, as described in "A Fabrication Technique for Multilayer Ceramic Modules", H. D. Kaiser et al., Solid State Technology, May 1972, pp. 35-40. Conductive molybdenum paste comprises molybdenum powder, having a predetermined average or mean particle size, suspended in an organic vehicle. The organic vehicle may include a binder resin, e.g., Ethyl Cellulose, a solvent, e.g., 2-(2-Butoxy ethoxy) Ethyl Acetate and a wetting agent or other additive, e.g., Oleoyl Sarcosine.
2. Background Art
The multilayer ceramic substrate fabrication process requires the viscosity of the molybdenum paste to be precisely controlled. Unfortunately, it was found that large viscosity variations exist in molybdenum pastes formed from molybdenum powders having the same average particle size and the identical vehicle. It was also found that in order to obtain the requisite paste viscosity, the vehicle viscosity may be changed by a trial and error procedure. For example, low and high viscosity vehicle may be formulated, e.g., with low and high molecular weight ethyl cellulose, respectively, and may be mixed in various proportions until the requisite paste viscosity is obtained. Such a trial and error process is time consuming and requires constant supervision because of its unpredictability. It was also found that changes in vehicle viscosity may produce other undesirable side effects in the resulting paste, e.g., excessive paste bleed. Accordingly, production of molybdenum paste having a predetermined viscosity from different lots of molybdenum powder having the same average particle size was heretofore a trial and error process which was not amenable to mass production.